1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus foaming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a fixing belt or a fixing film to apply heat to the recording medium bearing the toner image. FIG. 1 is a vertical sectional view of a fixing device 20R1 including such a fixing belt 204. The fixing belt 204 is looped around a heating roller 202 and a fixing roller 203, in a state in which a tension roller 206 biases the fixing belt 204. A pressing roller 205 presses against the fixing roller 203 via the fixing belt 204 to form a nip N between the pressing roller 205 and the fixing belt 204. The fixing belt 204 is heated by a heater 201 provided inside the heating roller 202. As a recording medium P bearing a toner image passes between the fixing roller 203 and the pressing roller 205 on the fixing belt 204, the fixing belt 204 and the pressing roller 205 together apply heat and pressure to the recording medium P bearing the toner image to fix the toner image on the recording medium P.
One problem with such an arrangement, however, is that the heating roller 202 has a relatively large heat capacity, resulting in a longer warm-up time for the fixing device 20R1. To address this problem, instead of the fixing belt 204 the fixing device may include a fixing film having a relatively small heat capacity. FIG. 2 is a vertical sectional view of such a fixing device 20R2 including a fixing film 213. A ceramic heater 211 is provided inside a loop formed by the fixing film 213. A pressing roller 212 presses against the ceramic heater 211 via the fixing film 213 to form a nip N between the pressing roller 212 and the fixing film 213. As a recording medium bearing a toner image passes between the pressing roller 212 and the fixing film 213, the fixing film 213 heated by the ceramic heater 211 and the pressing roller 212 together apply heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium.
However, the fixing film 213 also has a drawback in that, over time, friction between the ceramic heater 211 and the fixing film 213 sliding over the ceramic heater 211 increases, resulting eventually in unstable movement of the fixing film 213 and increasing the required driving torque of the fixing device 20R2.
Moreover, the temperature of the fixing device as the recording medium bearing the toner image enters the fixing device is critical to imaging outcome. In this respect, the fixing film 213 has another drawback in that the ceramic heater 211 heats the fixing film 213 at the nip N only, and therefore the rotating fixing film 213 is coolest when it reenters the nip N, resulting in formation of a faulty toner image on the recording medium due to the lower temperature of the fixing film 213 at that location.
To overcome these drawbacks, instead of the ceramic heater 211 the fixing device may include a heat generator provided inside the loop formed by the fixing film to heat the fixing film locally, and the temperature of the fixing film is detected by a temperature detector. However, there is a certain distance or a gap between the heat generator and the nip N in the direction of rotation of the fixing film, and the temperature detector is typically disposed in proximity to the heat generator. Accordingly, even if the temperature of the fixing film is controlled based on the temperature of the fixing film detected by the temperature detector disposed near the heat generator, the fixing film is still cooled when it enters the nip N. In other words, the temperature of the fixing film at the position where the heat generator faces and heats the fixing film directly may be different from the temperature of the fixing film at the nip N. As a result, a faulty toner image is formed on the recording medium due to the unstable fixing temperature of the fixing film at the nip N.